This invention relates to the determination of position and orientation for a mechanical structure relative to a proximal point in space. Preferred embodiments of the invention determine the co-ordinates of the human head when an individual is seated. This information enables head and torso movements of the individual to form part of his/her interaction with software being executed by a computer (Browse R A, Rodger J C, Sewell I and Brooke J: xe2x80x9cControlling graphic objects naturally: use your headxe2x80x9d: Proc. S.P.I.E. Int. Soc. for Optical Engineering, 3012, 448-53, 1997).
A variety of methods have been described for the determination of position and/or orientation of a rigid body. Typically these are based upon the use of optics, ultrasound [U.S. Pat. Nos. 4,682,159, 5,761,155, 5,495,427], mechanical linkages [U.S. Pat. No. 5,452,516], accelerometry [U.S. Pat. No. 5,615,132] and electromagnetism [U.S. Pat. Nos. 5,747,996, 4,622,644, 3,983,474]. To achieve satisfactory performance, or to correct for the drift associated with some methods, two or more methods may be combined [U.S. Pat. Nos. 5,645,077, 5,592,401]. For a review of helmet mounted head tracker technology, see Cameron A A, Trythall S and Barton A: xe2x80x9cHelmet trackersxe2x80x94the futurexe2x80x9d: Proc. S.P.I.E. Int. Soc. for Optical Engineering, 2465, 281-95, 1995.
A preferred embodiment of the present invention is optical, but the invention need not be restricted to this form of embodiment. Most prior systems have used at least three modulated light (or infra-red) sources and one [for example, U.S. Pat. No. 5,059,789, and Kim D, Richards S W and Caudell T P: xe2x80x9cAn optical tracker for augmented reality and wearable computersxe2x80x9d: Proc. I.E.E.E. 1997 Virtual Reality Ann. Int. Symp., Cat. 97CB36033, 146-50, 1997], or two [U.S. Pat. Nos. 5,717,201, 4,649,504] position detectors. The phenomenon of light diffraction has also been employed [U.S. Pat. No. 5,726,758]. In practice, more than three sources may be needed in an actual implementation based on triangulation principles. Prior sensor systems are typically capable of high accuracy [U.S. Pat. Nos. 5,452,516, 5,388,059] and of determining position and orientation of the rigid body in all six degrees of freedom [U.S. Pat. Nos. 5,059,789, 4,672,562, 4,649,504] but may need considerable pixel-based image processing [U.S. Pat. Nos. 5,608,528, 5,424,556, 5,353,042, 5,187,540].
A two-source system has previously been devised [U.S. Pat. No. 4,888,490], which projects two focused laser beams onto a uniform surface to determine the proximity and orientation of the surface with 4 degrees of freedom (orientation and proximity) relative to the laser sources. Retro-reflection from two pairs of co-incident sources and directional detectors have been described for computing positional information of passive reflectors based on directional information (U.S. Pat. No. 4,576,481). In another two-source device, a single photoquadrant detector is used to determine source directions, but not their intensity [U.S. Pat. No. 5,694,153]. Preferred embodiments of two other schemes [U.S. Pat. No. 5,510,893, UK Patent No GB 2 289 756 B or U.S. Pat. No. 5,627,565] may employ two sources and one photoquadrant-like detector. Again, however, the intensity of the sources is not exploited in the determination of relative position.
Preferred embodiments of the present invention aim to determine the co-ordinates of a moveable body, with up to 5 degrees of freedom, using only two diffuse light sources mounted on the moveable body. This is achieved by a combination of determining the direction of the sources, measuring the individual intensities of the sources and comparing their relative intensities.
According to one aspect of the present invention, there is provided a sensor of relative position and orientation, comprising:
a. first and second emitters which are mutually spaced by a predetermined distance and are arranged respectively to emit first and second electromagnetic radiation in respective first and second directions which have a mutual angular spacing of a predetermined angle; and
b. a receiver arranged to receive the electromagnetic radiation emitted by both of said emitters and to output:
i. a first signal dependent upon the total intensity of said first electromagnetic radiation as received by said receiver;
ii. a second signal dependent upon the total intensity of said second electromagnetic radiation as received by said receiver;
iii. a third signal dependent upon the direction of said first emitter relative to said receiver; and
iv. a fourth signal dependent upon the direction of said second emitter relative to said receiver:
the sensor further comprising:
c. signal processing means arranged to receive from the receiver said first, second, third and fourth signals and to derive therefrom, utilising a ratio of said first and second signals, an output signal indicating both the position of said emitters relative to said receiver and the orientation of said emitters relative to said receiver.
Preferably, said predetermined angle is other than 0xc2x0.
Preferably, said electromagnetic radiation is infra-red radiation.
Preferably, said receiver comprises a lens, an optical bandpass filter, an aperture and a detector.
Preferably, said receiver comprises a photoquadrant detector.
Said emitters may be mounted on a headset.
Said receiver may be mounted on a headset.
Said predetermined distance may be a fixed predetermined distance.
Said predetermined distance may be capable of limited variation.
Preferably, said receiver is fixed relative to world co-ordinates and said emitters are free to move.
Said emitters may be fixed relative to world co-ordinates and said receiver free to move.
Preferably, each of said emitters is arranged to emit its respective said electromagnetic radiation in such a pattern that contours of constant ratio of respective total intensities of said radiation at said receiver are orthogonal or near-orthogonal to contours of constant angular separation of said emitters subtended at said receiver.
Each of said emitters may be arranged to emit its respective said electromagnetic radiation in such a pattern that ratios of respective total intensities of said radiation at said receiver are substantially dependent upon the yaw of an imaginary line passing close to the optical centres of said emitters relative to said receiver.
Each of said emitters may be arranged to emit its respective said electromagnetic radiation in such a pattern that ratios of respective total intensities of said radiation at said receiver are substantially independent of the distance between said emitters and said receiver.
Each of said emitters may be arranged to emit its respective said electromagnetic radiation in such a pattern that ratios of respective total intensities of said radiation at said receiver are substantially independent of the rotation of said emitters about an imaginary line passing close to the optical centres of said emitters relative to said receiver.
Preferably, the radiation patterns of said emitters are oriented in different directions relative to each other.
Said position of said emitters relative to said receiver and said orientation of said emitters relative to said receiver may be determinable in respect of one or more or all of the following: lateral translation, longitudinal translation, yaw, pitch and roll.
Said position of said emitters relative to said receiver and said orientation of said emitters relative to said receiver may be determinable in respect of one or more or all of the following: lateral translation, vertical translation, longitudinal translation, yaw and pitch.
Said position of said emitters relative to said receiver and said orientation of said emitters relative to said receiver may be determinable in respect of one or more or all of the following: lateral translation, vertical translation, longitudinal translation, yaw, pitch and a mixture of vertical translation and pitch.
Preferably, said first and second electromagnetic radiation is coded by a pseudo-random code, or other near-orthogonal or orthogonal code, to allow independent, or near-independent, reception of said first and second electromagnetic radiation by said receiver.
Preferably, said first and second electromagnetic radiation is coded by a code that allows multiple further said emitters to be added and their respective radiations to be independently received by said receiver.
A sensor as above may further comprise one or more additional said emitter and/or one or more additional said receiver, to provide one or more further signal to said signal processing means to derive therefrom said output signal.
Said output signal may indicate both position and orientation with up to six degrees of freedom.
A sensor as above may further comprise a computational model of the physical sensor system which is used by said signal processing means for the determination of one or more co-ordinates estimated by said signal processing means from the signals received from said receiver.
A sensor as above may be adapted as an input device for a computer, to control operation of the computer.
The invention extends to a computer provided with a sensor according to any of the preceding aspects of the invention, arranged as an input device to control operation of the computer.